1. Field of the Invention
The present invention relates to downhole guide members employed in subterranean boreholes, typically drilled for oil and gas wells. More particularly, the invention relates to a downhole casing guide member comprising a uniquely contoured structure which guides multiple casing strings downhole and maintains such multiple casing strings in substantially fixed, side-by-side relationship with one another downhole within a conductor casing. The contour of the structure maximizes available cross sectional fluid flow area in a conductor casing, in a plane perpendicular to the center longitudinal axis of the conductor casing, to minimize fluid pressure losses when flowing fluids such as drilling fluids or cement slurries around the structure. Moreover, the contour of such structure, in the plane perpendicular to the longitudinal axis of the conductor casing, maximizes the diameter of the casing strings which may be run within a given conductor casing diameter by forming two bores, spaced substantially 180.degree. apart, where a portion of each bore is formed by a portion of the inner wall of the conductor casing. Additionally, the contour of the structure presents tapered upper and lower shoulders which ease insertion into and removal from the conductor casing.
2. General Background
In the development of certain oil and gas fields, it is at times highly desirable to drill multiple, directionally drilled wellbores from a common surface location inside a single large "conductor casing" string. This is especially so in certain offshore oil and gas developments. In the following description, the term "conductor casing" refers generally to the initial, generally large diameter casing string installed, through which multiple wells may be drilled. Conductor casing is typically driven, drilled, or jetted into place so that the lowermost end (the "shoe") is several hundred feet below the mudline.
After the conductor casing is in place, individual wellbores are drilled and the initial casing string of each individual well is run and cemented in place. In this description, the initial casing string for each individual well is referred to as "surface casing ". Although the process and present invention are described in terms of two surface casing strings run within a conductor casing, it is understood that the invention is not limited to arrangements comprising two surface casing strings and in fact comprises different numbers of surface casing strings. Typical arrangements employ two 13-3/8" surface casing strings run substantially side-by-side within a 36" conductor casing, although other combinations of conductor and surface casing diameters may be used and are within the scope of the present invention.
A typical sequence of operations is as follows: a 36" conductor string is driven into the earth so that the casing shoe is several hundred feet below the mudline, or ocean floor. As water depths may be several hundred feet, the total length of conductor casing may be on the order of 1000'. A large diameter drill bit, typically approaching the inner diameter of the conductor casing, is then used to drill out the conductor casing to a depth slightly beyond the conductor casing shoe. Thereafter, a sequence of installation of a downhole casing guide member in the conductor casing, drilling of surface casing holes, and running and cementing of two surface casing strings follows. The operations may vary depending upon the type of guide member used. Drilling, formation evaluation, running of additional casing strings, etc., in each borehole, then proceeds in generally conventional manner through each surface casing string.
It is important for the two surface casing strings to be held downhole in fixed, side-by-side spacing with respect to one another, and the downhole casing guide members serve this purpose. Fluid flow past the downhole casing guide members is necessary for passage of drilling fluids (commonly called "mud") and cement slurries during the drilling of the surface holes and the running and cementing of the surface casing strings. It is desirable, then, that the downhole casing guide member retain the surface casing strings in fixed side-by-side position while permitting use of the largest possible surface casing strings within a given conductor casing, and while occupying as little as possible of the available cross sectional fluid flow area within the conductor casing, thereby preserving relatively uninhibited fluid flow past the guide member. Further, as the guide member must be run into (and at times retrieved from) the conductor casing, a profile which minimizes "hanging up" on ledges, obstructions and the like is desired.
One such structure is described in U.S. Pat. No. 5,560,435, to Sharp, entitled "Method and Apparatus for Drilling Multiple Offshore Wells from Within a Single Conductor String." The invention, by Sharp, discloses a method of drilling multiple wells in a conductor casing string. The invention, by Sharp, uses a downhole drilling guide which is a cylindrical member having two opposing flat, planar surfaces, and includes multiple guide bores in a side-by-side, parallel alignment, for receiving a casing string in each guide bore. The guide member is installed by running it downhole on one surface casing string (secured on a releasable connector) until the guide member rests on an internal shoulder in the conductor casing. Surface casing strings are then run through the guide bores. The configuration of the guide member of Sharp results in little available flow area in the conductor casing when the guide member and surface casing strings are in place. Use of only a single guide member as taught by Sharp may make it difficult to properly guide the second surface casing string through the guide bore. In addition, the relatively abrupt shoulders of the Sharp apparatus would tend to "hang up" on ledges, obstructions and the like within the conductor casing string.
Another such structure is described in U.S. Pat. No. 5,458,199, to Collins et al., entitled "Assembly and Process for Drilling and Completing Multiple Wells." The invention, by Collins et al., uses a downhole tie-back assembly to maintain the casing strings separated while downhole. The downhole tie-back assembly comprises bores for running the casing strings therethrough. In one embodiment, a first casing string is is threaded into screw threads in the first bore, and the first casing string is used to lower the tie-back assembly into place. A collet latch is attached to the exterior of the second casing string, and that collet latch snaps into a mating profile in the second bore, thus connecting the second casing string to the tie-back assembly. Relatively little flow area remains in the conductor casing with installation of the tie-back assembly and the two casing strings. Additionally, the maximum size of casing strings that may be run within the conductor casing is reduced due to the tie-back assembly bores completely encompassing the casing strings, and the relatively abrupt shoulders of the Collins et al assembly may result in hang-ups on interior ledges, etc. in the conductor casing during running.
As can be appreciated, the known downhole casing guide member structure and tie-back assemblies require at least one of the surface casing strings to be latched or secured to the downhole guide member and tie-back assembly to lower such downhole guide member downhole. In effect, one of the surface casing strings serves as the "running string" for the downhole guide member. Moreover, once both casing strings are installed in the downhole guide members of known design, the remaining fluid flow area around the downhole guide member and/or tie-back assembly and the first and second surface casing strings is insufficient for easy flow of displaced drilling fluids and/or cement slurries around the downhole guide member and/or tie-back assembly. Although the maximum diameter of casing strings that may be run within a conductor is (in the case of two casing strings) fundamentally limited to one-half of the inner diameter of the conductor, with the downhole casing guide members of known design the maximum outer diameter of the two casing strings is further limited by the bores in the downhole guide member being completely contained within the diameter of the downhole guide member. The abrupt upper and lower shoulders on the downhole guide members of known design do not permit easy passage past ledges or obstructions in the conductor casing.